Selenium is an essential trace element which has provoked considerable interest due to the recent identification of prokaryotic and eukaryotic proteins that contain the amino acid, selenocysteine. Incorporation of selenocysteine into these proteins requires a novel translation step in which UGA specifies selenocysteine insertion. Since UGA condons are typically recognized as translation stop signals, a number of intriguing questions are raised. The specific aims of this proposal are 1) to investigate the series of events that take place when a ribosome encounters a UGA selenocysteine condon, 2) to investigate what factors affect the efficiency of stop condon recognition in eukaryote, and to determine how and to what extent these factors come into play in circumventing termination at eukaryotic selenocysteine condons, and 3) to investigate how differences in eukaryotic and prokaryotic sleenocysteine incorporation mechanisms affect the efficiency of incorporation versus termination. The identification of selenocysteine in several mammalian proteins during the last few years has provided new insights into the functions of this trace nutrient. All of the selenoenznes identified to date catalyze oxio-reduction reactions in which the selenocysteine residue is in the active site. Studies of selenocyseine incorporation in type 1 iodothyronine deiodinase opened the door for investigation of the requirements for eukaryotic sleenoprotein synthesis and the features that distinguish this pathway from the mechanism in prokaryotes. While significant progress has been made in recent years, investigation of the complex interplay between the mRNA secondary structures, putative selenocysteine- specific elongation factor, and other components of the translations machinery remains foremost among the tasks ahead. Examination of physiological circumstances contributing to tissue-specific differences in selenocysteine incorporation will help to explain how the animal has adapted to maximize utilization of this trace element, with crucial implications for advancing our understanding of the roles of selenium in health and disease. Finally, studies of eukaryotic selenprotein synthesis have provided unexpected insights into the mechanisms of translation and termination of protein synthesis. Continuing studies of this process will surely unveil new and exciting information about the complex workings of the translation machinery.